Arnold Schwarzenegger and Sylvester Stallone had an intense rivalry back in the day ... but now they are good friends and brothers in arms ... and now's your chance to understand how this change came about.

"TMZ Presents: Arnold & Sly: Rivals, Friends, Icons" is our exclusive interview with the two Hollywood icons, where they discuss how they went from competitors to pals in a sometimes stunning conversation with Harvey Levin .

The full documentary is now available on YouTube for the first time ever.

In the interview, Sly tells us he came within a hair's breadth of tanking "Rocky II" because of a gruesome injury, and he recalls how he powered through to shoot the flick. Sly says pushing through pain is what driven folks like Arnold and himself did to make it to the top. P

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